If gravity itself isn't a fundamental force, but rather an emergent ones, many of the mysteries of space and time may have a different solution than the ones we're presently seeking. Image credit: Zoltán Vörös of flickr.

“In our description of nature the purpose is not to disclose the real essence of the phenomena but only to track down, as far as possible, relations between the manifold aspects of our experience.” -Niels Bohr

When it comes to galaxies, clusters of galaxies or the largest-scales on the Universe, dark matter is absolutely required. There’s no way to explain colliding galaxy clusters, correlations between different galaxies, the filamentary structure of the Universe or the fluctuations in the Cosmic Microwave Background without it. But it isn’t just dark matter that hopes to solve these problems, and there’s a new contender in town: emergent gravity.

The dark matter halo around galaxies could be explained, in principle, by a new type of entropy that’s affected by the normal, baryonic matter present in space. Image credit: ESO / L. Calçada.

20 years ago, Ted Jacobson proposed, after noting similarities between gravitation and thermodynamics, that gravity might be an emergent phenomenon. Erik Verlinde has run with it, producing a formalism to get dark matter and dark energy-like effects out of a Universe where a new kind of entropy exists. Recently, it’s made predictions for galactic rotation curves, and with marginal successes, may be poised to test those larger scales.

The fluctuations in the Cosmic Microwave Background, or the Big Bang’s leftover glow, contain a plethora of information about what’s encoded in the Universe’s history. Image credit: ESA and the Planck Collaboration.